Generally, light emitting diodes should be driven by a constant current in order to decrease variations or deviations in the intensity of the output of the diodes. There is a known driving circuit comprising a plurality of resistors each connected in series with an integrated driving FET and a light emitting diode and having resistance which is much larger than the resistance during the conducting state of the light emitting diode. According to this circuit, the value of current flowing through the light emitting diode is substantially determined by the above-mentioned series resistor and variations or deviations in current caused by resistance variations or deviations of the driving FETS and light emitting diodes are suppressed, whereby a substantially constant current operation can be obtained.
However, the foregoing driving circuits have some problems as follows.
1. Since a plurality of resistors corresponding in number to the number of light emitting diodes are connected outside the IC device, the number of circuit parts forming an electronic circuit system is increasing the cost due to an increase in the number of assembly steps.
2. The resistors dissipate a large amount of uneffective current and voltage. This causes a problem especially for a pocket calculator which is powered by a battery.